S.42 Medium
Your boiler has temporarily paused because a safety flap in the flue system is not providing the correct signal to allow the burner to start.
Technical description: Flue non return flap return signal blocks burner operation
Typical repair cost: £120-250 (including labour)
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Protect yourself from future repair bills
A boiler cover plan would cover faults like S.42 — no call-out fees, no surprises.
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Parts commonly replaced
- Flue non-return flap
- Flue gas sensor
- Wiring harness
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